Prior art CMOS Active Pixel Sensor pixel architectures are shown in FIGS. 1, 2 and 3. Referring to FIG. 1, this four-transistor pixel has a photodetector n-PD and a charge-to-voltage conversion region n+ in each pixel. This architecture provides the some advantages. The first advantage is global shutter operation by using the charge-to-voltage conversion region n+ as a storage region. The second advantage is pixel symmetry. Each pixel is identical which provides identical photoresponse for each pixel. This pixel architecture also has disadvantages. A first disadvantage is that for large pixel sizes, the capacitance of the charge-to-voltage conversion region can be too small to hold all of the charge that is accumulated in the photodetector. A second disadvantage is lack of in-pixel charge domain binning.
The pixel shown in FIG. 2 addresses the first disadvantage. The charge-to-voltage conversion region capacitance is increased by incorporation of an added capacitance Cadd in each pixel. While this solves the charge-to-voltage conversion region capacitance, and can provide a more linear output response, it has the disadvantage of reducing the fill factor of the pixel due to the area in the pixel allocated to the added capacitance Cadd. The reduced fill factor, adversely affects the charge capacity and dynamic range of the pixel.
The pixel shown in FIG. 3 solves the disadvantages of lack of charge domain binning capability, low charge-to-voltage region capacitance, and low fill factor by sharing the charge-to-voltage conversion n+ between adjacent pixels. This pixel architecture has other disadvantages. A first disadvantage is pixel symmetry. Each pixel is not identical, and this can lead to systematic differences in photoresponse, causing fixed pattern noise that can degrade image quality. A second disadvantage is lack of global shutter capability. Since the charge-to-voltage conversion region is shared by adjacent pixels, it cannot be used as a charge storage region to isolate the signal captured by individual photodetectors.
Consequently, a need exits for an image sensor that overcomes the above-described drawbacks.